How to straighten a (small) tree

I have a standard olive tree in my garden, about 2.2m tall with 10cm diameter trunk.

Last winter, while the ground was sodden and the wind blowing strong it developed a list of 10-15 degrees. Root ball has not lifted, nothing broken and tree seems healthy - but how can I het it back near vertical again?

Don't knock it in vertically, put it at 45 degrees. That means that when you brace the tree and it moves in the wind it will push against the pole pushing it in. if you do it vertically it loosens off and will not bring the tree back to vertical. You attach the tree to the pole with an old inner tube so that it dos not constrict the growth and will perish with in a few years, by which time it should be back vertical. The tree will naturally tend to go vertical over time, you are just making sure that it does it quicker and that any loosening of the root ball does not cause further movement.